As is presently known, CDMA 2000 as the third-generation cellular system is in need of a larger system capacity to transmit high-speed packet data as well as audio data. Such extension of system capacity confronts several obstacles involved in inherent properties of wireless communication systems. It is well known that the most important factor in mobile communication is to reduce “fading” that causes distortions of received signals.
In overcoming fading, it is preferred to employ diversity techniques. The diversity is generally used to combat multi-path fading, being applied on both the transmission and reception sides. There are essentially three kinds of diversity: time, frequency, and space.
In the CDMA 2000 system, a forward link governs system capacity because it has a smaller channel capacity than a reverse link, in contrast to the second-generation cellular system. Those conditions arise from the fact that the forward link is available to apply the maximal ratio coupling to a reception signal on the reception diversity employing two antennas at a base station. Since applying the reception diversity of two antennas burdens a terminal, the CDMA 2000 system employs the base station transmission diversity so as to balance the difference between channel capacities of the forward and reverse links.
There have been proposed several kinds of transmission diversity techniques, such as OTD (orthogonal transmission diversity), TSTD (time-switched transmission diversity), STD (selection diversity), TXAA (transmission antenna array), and so on. It would not be apparent to discriminate superiority and inferiority between the diversity techniques about which one is capable of facilitating an optimal trade-off in view of functional enhancement in comparison with implementation complexity. It may be general to choose the best way among them in accordance with a given communication environment.